The invention relates to a method and a system for symbol time error estimation in a broadband transmission system.
The invention is preferably used in data transmission systems employing orthogonal frequency division multiplexing (OFDM), in particular in wireless applications for digital video broadcasting (DVB, e.g. DVB-H, DVB-T), but can also be used for other transmission modes, such as ISDB-T, DAB, WiBro and WiMax. DVB, e.g. DVB-H and DVB-T are known standards for bringing digital television content for instance to mobile devices.
Such orthogonal frequency division multiplexing systems are very sensitive to the intersymbol interference (ISI), which is caused by the loss of orthogonality of the symbols. The invention relates to the compensation of the intersymbol interference by estimating a symbol time error.
The orthogonal frequency division multiplexing mode is a mode which converts a stream of symbols in a frame into parallel data of a block unit and then multiplexes the parallel symbols into different sub-carrier frequencies. The multi-carrier multiplex has the property that all carriers are orthogonal to one another with respect to a certain length that is typically 2n such that a fast Fourier-transformation can be used. The OFDM mode is implemented with the discrete Fourier-transformation (DFT) at a receiver and the inverse discrete Fourier-transformation (IDFT) at a transmitter, which is simply obtained by the orthogonal property and the definition of the discrete Fourier-transformation.
In broadband transmission systems, a guard interval is formed by a cyclic extension preceding the output of the inverse discrete Fourier-transformation for each OFDM symbol.
FIG. 1 shows the conventional structure of an OFDM symbol that is protected by a guard interval. The guard interval is formed by a cyclic prefix, i.e. a copy of the last samples of the so-called useful part is preceding the useful part. If there is no multipath, the receiver can select a window that is the size of the useful part anywhere within this symbol as shown in FIG. 2.
The guard interval protects the useful data carrying part from multipath distortion, and, if chosen sufficiently long, allows for single frequency networks (SFN). In an SFN, multiple transmitters transmit the same signal synchronously such that at a receiver those signals can be treated as multipath signals.
In multipath propagation environments, a transmitted signal reaches the receiver through multiple paths each of which may introduce a different delay, magnitude and phase thereby enlarging the transition time from one symbol to the next. If the transition time is smaller than the guard interval, the receiver can select a portion of the received symbol that is free from any interference introduced by adjacent symbols.
Identifying the useful part, i.e. the part of an OFDM symbol that contains minimum interference from adjacent symbols (intersymbol interference), is a time synchronization task to be performed by the receiver. This task is critical to the overall receiver performance.
Time synchronization can be grouped into two main categories: acquisition and tracking. Symbol time acquisition defines the task of initially finding the correct timing. Often, the symbol time acquisition is divided into two or more steps, where in the first step, coarse time synchronization is achieved. In the following steps, the time window is refined. For those successive steps, similar or identical algorithms that are used for tracking are often applied. Tracking defines the task of constantly adjusting the time window in the course of continuous reception to keep the time window in its optimum location.
For OFDM, many efforts have been made for time tracking. The known methods can be grouped into data assisted and non-data assisted tracking, and pre-FFT or post-FFT based tracking. Data assisted tracking makes use of known symbols in OFDM, e.g. pilot symbols or preambles, where non-data assisted tracking makes use of the correlation properties of the signal.
In DVB-T which is aimed at continuous reception, the standard does not define any preambles. Pilot symbols are included in the multiplex, where the standard defines so-called scattered pilots at every 12th carrier, and a smaller number of continual pilots that are present at fixed carrier locations.
The conventional insertion of the scattered pilots that are boosted in power as described in FIG. 11, on page 27 of European Telecommunication Standards Institute ETSI EN 300 744 V 1.4.1 (2001-01).
Those pilot symbols are only accessible after the DFT and only after some coarse time synchronization has already been established. Therefore, most time synchronization algorithms for DVB-T/H use the auto-correlation properties of the OFDM symbols with its cyclic extension for coarse symbol time estimation, and then rely on the pilots for fine time synchronization and tracking.
In DVB-T the guard interval can be selected to be ¼, ⅛, 1/16, or 1/32 of the FFT (or DFT) size. In large scale single frequency networks (SFNs) even a guard interval of ¼ of the FFT size can almost be fully used by multipath. In some cases, it has been found that the delay spread even exceeds the guard interval. With pilots at every 12th carrier, a channel impulse response of a time span of only 1/12 of the FFT length can be estimated which is clearly not sufficient for guard intervals equal or greater than ⅛. For reliable time synchronization for guard intervals equal to ⅛ of the FFT size or longer, it is therefore necessary to collect pilots from successive symbols in the same or similar fashion as it is done for estimating the channel transfer function that is needed for the frequency domain equalizer.
Two basic approaches for post-FFT based time synchronization are known both using an estimate of the channel transfer function: The first one calculates the average phase difference from one scattered pilot to the next thereby estimating the mean slopes of the channel transfer function. This is based on the property of the FFT that a delay in time domain corresponds to a phase proportional to the carrier index and proportional to the delay in time domain. Therefore, in single paths channels, the time delay, which is denoted as τ in FIG. 2, can be directly estimated from the slope. Unfortunately, this technique does not perform satisfactorily under heavy multipath conditions. The more rigorous approach is to transfer the estimated channel transfer function back into time domain by means of an IFFT to obtain an estimate of the channel impulse response. Afterwards an energy search is performed on the estimated channel impulse response.
Another known approach is based on the continual pilots only.
A known alternative to post-FFT based time synchronization is to further improve the time domain correlation based method typically used for coarse time synchronization.
As discussed above, time tracking is crucial for the overall system performance. In DVB-T/H, the lack of preambles that could help accurately estimate the channel impulse response makes it difficult to find the optimum time window.
Some pre-FFT time domain based time tracking techniques that make use of the auto-correlation properties have been found to require relatively long averaging times to yield adequate results. Another disadvantage is that after the signal has been acquired; those types of calculations are not required elsewhere in the receiver. Additionally, the performance under heavy multipath is not always optimum.
The post-FFT based methods introduced above also have disadvantages.
As said above, the simple method using the estimate of the mean value of the slope of the channel transfer function, albeit giving satisfactory results in channels with low delay spread, has been found not to give adequate results under heavy multipath conditions as can be experienced in SFNs. Experiments have shown that this method does not withstand tests for guard interval utilization in single frequency networks.
The most robust technique up to now seems to be the IFFT based method, which calculates the channel impulse response from the estimated channel transfer function. This method, however, also is the most computational intensive method and requires additional memory. The problem that needs to be overcome when using this type of algorithm is the cyclic wrapping of the channel impulse response after ⅓ the FFT length that is due to the scattered pilot spacing at every third carrier when multiple symbols are collected. The cyclic wrapping may make it difficult to identify the beginning and end of the channel impulse response. Identifying the impulse response is also difficult in noisy environments, when the energy of the impulse response is spread over a large time interval.
DVB-H designed for mobile reception imposes additional challenges on the symbol time synchronization algorithms:    (1) In a mobile environment, the coherence time of the channel is lower, i.e. the channel is more time-varying.    (2) DVB-H makes use of time slicing. In time slicing, data are transmitted in bursts allowing the receiver to be switched off between bursts. This feature that allows the receiver to save a great deal of power consumption, however, also means that the channel cannot be tracked between bursts.
As a merit, the time tracking algorithms for DVB-H must be substantially faster than for DVB-T.
To illustrate those challenges, the following example of a two-path model as used in a test case is considered.
FIG. 3 shows the magnitude of the impulse responses of the conventional two path model at two timing instances, t1 and t2, respectively. The two paths are separated by 0.9 times the guard interval duration Tg. At time instant t1, the second path is not really visible, as it is faded. In the real world, the first path may originate from one transmitter, and the second from another transmitter. Both transmitters synchronously transmit the same signal on the same frequency (SFN). At time instant t1, the second path is not visible as it can be blocked by an obstacle (shadow fading) or the path is actually a superposition of multiple paths that at time instance t1 add destructively (fast fading). A receiver locking to a received signal that experienced this channel at time instance t1, only sees the first path, and may just center this path to the middle of the guard interval. If the receiver is synchronizing to the signal to receive time-sliced bursts, it essentially has no history on the channel to rely on.
When after a relatively short time, e.g., a couple 10 ms the second path occurs, the receiver has to quickly readjust the symbol timing and place both paths into the guard interval such that no intersymbol interference occurs in the useful part.
Likewise, it is also possible that at time instance t1, the first path was subject to fading, and the receiver initially locked onto the second path.
This example shows that the symbol time tracking requirements for DVB-H are much more stringent than for continuous reception especially in stationary or quasi-stationary environments.
For DVB-T, it has often been argued that the computational load of the IFFT based method can be reduced, since the symbol time tracking can be done on a lower rate, and thus an IFFT does not have to be computed for every received symbol. In the context of mobile DVB-H, i.e. rapidly time varying channels and fast reacquisition times to reduce on-times and therefore power consumption, this assumption does not hold.